Rugate filters are interference filters which block transmission of light at selected wavelengths, and can be used for many purposes. Among the various uses are glasses, for example, for protection against laser light. In some cases, the rugate filter may have to be applied to a curved transparent surface, as for example the curved surface of a pair of glasses. Functional effectiveness requires that the coating or coatings which make up a rugate filter have thickness which is uniform, in a three-dimensional sense, across the optical aperture being protected. That is, the thickness of the rugate filter coating(s) must be uniform at each and every point over the three-dimensional surface of the lens or glass.
A common method for application of rugate filter coatings is plasma-enhanced chemical vapor deposition (PECVD). A problem with the fabrication of rugate filter coatings on curved dielectric surfaces is that the electric fields which generate the plasma for enhancement tend to be distributed in a nonuniform manner across the surface being coated, which in turn tends to adversely affect the coating uniformity.
Improved coating methods are desired.